Displacement sensing system and method

ABSTRACT

A displacement sensing system and method addresses demanding requirements for high precision sensing of displacement of a shaft, for use typically in a linear electro-dynamic machine, having low failure rates over multi-year unattended operation in hostile environments. Applications include outer space travel by spacecraft having high-temperature, sealed environments without opportunity for servicing over many years of operation. The displacement sensing system uses a three coil sensor configuration, including a reference and sense coils, to provide a pair of ratio-metric signals, which are inputted into a synchronous comparison circuit, which is synchronously processed for a resultant displacement determination. The pair of ratio-metric signals are similarly affected by environmental conditions so that the comparison circuit is able to subtract or nullify environmental conditions that would otherwise cause changes in accuracy to occur.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract No.DE-AC03-02SF22491 awarded by the United States Department of Energy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to systems that sense whendisplacement occurs and, more particularly, to a displacement sensor forlong term unattended operation under demanding environmental conditions.

2. Description of the Related Art

Linear electro-dynamic machines convert linear motion of a shaft intoelectrical power and are used for and have further potential fordemanding applications such as unattended operation of space craft powersystems, other remote systems in hostile environments, and otherimplementations including linear coolers and linear drives. Duringoperation, certain displacement information regarding the shaft isnecessary. Unfortunately, conventional methods for sensing displacementhave not been able to meet requirements raised such as for highprecision sensing having low failure rates over multi-year unattendedoperation in hostile environments. Furthermore, imprecise displacementsensing can adversely affect related and other systems includingStirling Engine systems, down-hole oil pumps, and free-piston engines ingeneral.

BRIEF SUMMARY OF THE INVENTION

The present invention resides in a displacement system and method with ashaft having magnetic permeability greater that of a medium, which atleast surrounds a portion of the shaft. The shaft is configured forbi-directional linear travel along a first axis. Included is a drivesignal generator including an oscillator configured to generate a drivesignal having a periodically varying parameter and a drive coil havingturns of wire, the drive coil electrically coupled to the drive signalgenerator to be energized by the drive signal generator to produce atime varying magnetic flux. Included is a reference coil having turns ofwire, the reference coil being in proximity of the drive coil for afirst parameter to be induced in the reference coil by the magneticflux.

Included is a sense coil having turns of wire having a longitudinal axisin line with the first axis such that at least a portion of the shaftpasses into the sense coil to magnetically engage the sense coil duringa least a portion of the bidirectional linear travel of the shaft, thesense coil being in proximity of the drive coil for a second parameterto be induced in the sense coil by the magnetic flux, the amplitude ofthe second parameter being influenced by how far the shaft has advancedinto the sense coil, the ratio of magnitude of the second parameter tothe first parameter being known for at least one position of advancementby the shaft into the sense coil during the bi-directional linear travelof the shaft. Also, included is a signal processor configured to comparethe first parameter to the second parameter to identify when the shaftis at the at least one position of advancement.

Other features and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic of an implementation of a displacement sensingsystem according to the present invention.

FIG. 2 is an isometric view of a cylinder assembly, which is part of acoil assembly of the implementation of the displacement sensing systemof FIG. 1.

FIG. 3 is a schematic of an implementation of the displacement sensingsystem with a signal processor including a microcontroller.

FIG. 4 is a schematic of an alternative implementation of thedisplacement sensing system with a multiplexer.

DETAILED DESCRIPTION OF THE INVENTION

As will be discussed in greater detail herein, a displacement sensingsystem and method addresses demanding requirements for high precisionsensing of displacement of a shaft, for use typically in a linearelectro-dynamic machine, having low failure rates over multi-yearunattended operation in hostile environments. Applications include outerspace travel by spacecraft having high-temperature, sealed environmentswithout opportunity for servicing over many years of operation, andother environments not necessarily remote or having high temperature andother implementations such as involving linear coolers and lineardrives.

Conventional systems have not been able to meet the challenge ofunattended long term precision displacement sensing (such as within0.002 inches) in hostile environments. Optical sensors have not beenable to tolerate high temperatures (over 100 C) for long periods. Also,magnetic sensors, such as Hall Effect sensors, have been unable toprovide sufficient accuracy. Further, conventional coil type sensorshave failed to maintain their accuracy over changes in environmentaltemperature, which is a common occurrence with hostile environments suchas with spacecraft traversing outer space and with otherimplementations.

The displacement sensing system and method described herein uses a threecoil sensor configuration, including a reference coil, to provide a pairof ratio-metric signals, which are inputted into a synchronouscomparison circuit for a resultant displacement determination. The pairof ratio-metric signals are similarly affected by environmentalconditions so that the synchronous comparison circuit is able tosubtract or nullify environmental conditions that would otherwise causechanges in accuracy to occur. The displacement sensing system and methodalso can typically handle very high temperatures and ambient pressuressuch as those exceeding 500 psia. Implementations can even be used inhot, nonconductive, non-corrosive fluids.

An implementation according to the present invention of a displacementsensing system 100 is shown in FIG. 1 as including a drive signalgenerator 110, a clock signal conditioner 112, a coil assembly 114, anda signal processor 116 for sensing one or more aspects (such asdisplacement, velocity, acceleration, jerk, phase, etc.) of a shaft 118having an axis of bi-directional linear travel 120. In someimplementations, the signal processor 116 could also be configured tocontrol a process or system based upon the determination of the sensedaspect of the shaft 118. The shaft 118 is either part of or coupled toanother shaft (not shown) of a linear motive device 122 such as a linearelectro-dynamic machine as described above. As depicted, the coilassembly 114 and the linear motive device 122 are shown as separatestructures, however, in other implementations, the coil assembly couldbe structurally integrated with the linear motive device.

In the depicted implementation, the drive signal generator 110 includesan oscillator 130 that generates a drive signal of periodic form such asa sine wave, a square wave, or other periodic form. Accuracy of thedisplacement sensing is generally not dependent upon the frequency ofthe periodic signal so such frequency can be conveniently chosen basedupon available components. In the depicted implementation, the drivesignal generator 110 further includes a filter 132 for furtherconditioning of the drive signal. Other implementations do not includethe filter 132. The coil assembly 114 includes a reference coil 140 ofwire, a drive coil 142 of wire, and a sense coil 144 of wire that sharea common longitudinal axis being the axis of bi-directional lineartravel 120 of the shaft 118. In the depicted implementation, the shaft118 has a magnetic permeability relative to a vacuum typically greaterthan 10, such as greater than 20. Generally, the shaft 118 will be atleast partially surrounded by a medium such as air, vacuum, oil, orother gas or liquid and the magnetic permeability of the shaft will begreater than the medium. The shaft 118 passes fully through thereference coil 140 and the drive coil 142 for a first degree of magneticengagement with these coils. The shaft 118 also passes through the sensecoil 144 to magnetically engage the sense coil in the depictedimplementation less than or equal to the first degree of engagement ofthe shaft with the reference coil 140 and the drive coil 142. In otherimplementations the magnetic engagement of the sense coil may have othervalues.

The drive signal generator 110 is electrically coupled to a first end ofthe drive coil 142 with the second end of the drive coil coupled toground such that the drive coil is energized when the drive signal isgenerated. This configuration of the drive signal generator 110 and thedrive coil 142 produces a time varying magnetic flux to induce a timevarying voltage across the reference coil 140 and the sense coil 144. Asshown, the drive coil 142 is positioned between the reference coil 140and the sense coil 144, which helps more evenly distribute the magneticflux between the sense coil and the reference coil and minimizespotential interference between the coils. Voltage of the drive signaland voltages across the reference coil 140, the drive coil 142, and thesense coil 144 are the depicted parameters discussed herein, however,other implementations use other parameters such as current of the drivesignal and currents through the reference coil, the drive coil and thesense coil.

In many of the implementations, the sense coil 144 has a number of turnsof wire that is greater than the number turns of wire that the referencecoil 140 has. In some implementations, the drive coil 142 has a numberof turns of wire that is fairly comparable to the number of turns ofwire of the sense coil 144. A null position exists related to travel ofthe shaft 118 such that when the shaft is at this null position, thetime varying magnetic flux generated by the drive coil 142 will inducethe same voltage difference across the reference coil 140 as across thesense coil 144. During calibration procedures, this null position forthe shaft will be determined so that during operation, the null positionis a predetermined position that can be used in part in determiningcurrent position of the shaft 118.

In other implementations, the number of turns of the reference coil 140,the drive coil 142, and the sense coil 144 can be different and theordering of the coils can also be different. With these implementations,the ratio of amplitudes of the voltage (or other parameter) across thereference coil 140 and the sense coil 144 (such as a ratio of unity) atthe point in which the shaft 118 reaches a predetermined position (suchas the null position) is determined during a calibration period.Furthermore, the value for this ratio of voltage amplitudescorresponding to the predetermined position of the shaft 118 isrepeatedly consistent over wide ranges of temperature since changes inmagnetic coupling of the shaft with the reference coil 140 due tovariations in temperature will be nearly identical to the correspondingchanges in magnetic coupling of the shaft with the sense coil 144.

In the depicted implementation, the ratio of voltage amplitudes acrossthe reference coil 140 and the sense coil 144 is approximately unitywhen the shaft 118 reaches the predetermined position (its nullposition). In other implementations, the ratio could be something otherthan unity. At any position of the shaft 118, the ratio of the voltages,along with the known null position, can be used to determine position ofthe shaft.

In order to compare the voltage across the reference coil 140 with thevoltage across the sense coil 144, the two coils both have first endscoupled to inputs of a synchronous comparison circuit 146 of the signalprocessor 116 and both have second ends coupled to ground. As depicted,the output of the comparison circuit 146 is inputted to a D flip-flop148, which is also part of the signal processor 116. The clock input(CLK) of the flip-flop 148 is coupled to the clock signal conditioner112.

The clock signal conditioner 112 has a phase shifter 150, which isconfigured to delay the drive signal originating from the drive signalgenerator 110 by 90°. The clock signal conditioner 112 further has asynchronous comparison circuit 152 coupled to the phase shifter 150. Thecomparison circuit 152 is referenced to ground such that the comparisoncircuit squares the delayed signal outputted by the phase shifter 150.Consequently, for the depicted implementation, when the drive signalgenerator 110 generates a drive signal, the clock signal conditioner 112properly clocks the flip-flop 148 so that the signal processor 116outputs a logical “one” signal when the shaft 118 reaches its nullposition during travel of the shaft. It is anticipated that otherimplementations of the clock signal conditioner 112 could also be used.

In the depicted implementation, the coil assembly 114 further includes acylinder assembly 160. In this depicted implementation the cylinderassembly 160 has first, second, third, and fourth cylinder sections 162a-d, as shown in FIG. 2. In other implementations, the cylinder assembly160 can be otherwise configured and is not limited to that shown in FIG.2. The external surfaces of the cylinder sections 162 a-d arepartitioned by spaced-apart first, second, third, and fourth dividers164 a-d such that the external surfaces of the first and second cylindersections 162 a and 162 b are partitioned by the first divider 164 a, theexternal surfaces of second and third cylinder sections 162 b and 162 care partitioned by the second divider 164 b, the external surfaces ofthird and fourth cylinder sections 162 c and 162 d are partitioned bythe third divider 164 c, and the external surface of the fourth cylindersection 162 d is also partitioned by the fourth divider 164 d spacedapart from the third divider 164 c.

The cylinder sections 162 a-d share a common internal cylindrical volumehaving an opening 166 to receive the shaft 118 therein. In someimplementations the cylinder sections 162 a-d can be individual piecesjoined together with the dividers 164 a-c acting as flanges. In otherimplementations, the cylinder sections 162 a-d can be part of a singlecylinder where the dividers 164 a-d are coupled along appropriatepositions to the external surface of the single cylinder. For thedepicted implementation, the reference coil 140 is wound around theexternal surface of the second cylinder section 162 b, the drive coil142 is wound around the external surface of the third cylinder section162 c, and the sense coil 144 is wound around the external surface ofthe fourth cylinder section 162 d.

In other implementations of the signal processor 116, the ratio ofvoltage across the sense coil 144 compared with the reference coil 140is either measured for various positions of the shaft 118 by amicrocontroller. The microcontroller 172, shown in FIG. 3, having analogto digital (A/D) inputs and a digital input, perfoms the measurements atthe transition of the digital input, which is the synchronous clockingsignal. The microcontroller can then calculate the ratio of voltageacross the sense coil 144 compared with voltage across the referencecoil 140 for any given position of the shaft 118. The microcontrollercould then output the ratio calculation digitally or via a digital toanalog (D/A) output thereby reporting position of the shaft 118 basedupon the calculated ratio. The position report from the sensor can befurther used to derive other parameters such as velocity, acceleration,jerk, phase, etc.

In another implementation shown in FIG. 4, the voltages from thereference coil 140 and the sense coil 144 are inputted to an analogmultiplexer (MUX) 182 (such as a dual 2-to-1 multiplexer, multipleanalog multiplexers, multiple analog switches, field effect transistors(FETs) used to effect analog switching, or other means), which issynchronously switched in phase with the voltage from the reference coil142 by a synchronous comparison circuit 180. This achieves synchronousrectification of both signals. The outputs from the MUX 182 drive adifference amplifier 184 and filter 186, which produces an outputvoltage that is linear with stroke position.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A system comprising: a shaft having magnetic permeability greaterthan that of a medium that at least partially surrounds the shaft, theshaft configured for bi-directional linear travel along a first axis; adrive signal generator including an oscillator configured to generate adrive signal having a periodically varying parameter; a drive coilhaving turns of wire, the drive coil electrically coupled to the drivesignal generator to be energized by the drive signal generator toproduce a time varying magnetic flux; a reference coil having turns ofwire, the reference coil being in proximity of the drive coil for afirst parameter to be induced in the reference coil by the magneticflux; a sense coil having turns of wire having a longitudinal axis inline with the first axis such that at least a portion of the shaftpasses into the sense coil to magnetically engage the sense coil duringa least a portion of the bi-directional linear travel of the shaft, thesense coil being in proximity of the drive coil for a second parameterto be induced in the sense coil by the magnetic flux, the amplitude ofthe second parameter being influenced by how far the shaft has advancedinto the sense coil, the ratio of magnitude of the second parameter tothe first parameter being known for at least one position of advancementby the shaft into the sense coil during the bi-directional linear travelof the shaft; and a signal processor configured to compare the firstparameter to the second parameter to identify when the shaft is at theat least one position of advancement.
 2. The system of claim 1 whereinthe parameter of the drive signal is a periodically varying voltage, thefirst parameter is a voltage across the reference coil, and the secondparameter is a voltage across the sense coil.
 3. The system of claim 1further comprising an elongated member with a longitudinally extendingbore therein, the cylinder having an internal volume, an externalsurface, and dividers protruding from the external surface, thereference coil, the drive coil, and the sense coil being wound aroundthe external surface of the elongated member at longitudinally spacedapart positions and being separated from each other by the dividers, theshaft being reciprocally mounted in the bore and positioned at variousdegrees of advancement into the bore of the elongated member during thebi-directional linear travel of the shaft.
 4. The system of claim 3wherein the drive coil is positioned along the elongated member betweenthe reference coil and the sense coil.
 5. The system of claim 1 whereinthe parameter of the drive signal generated by the drive signalgenerator periodically varies according to a one of the followingfunctions: a square wave function and a sine wave function.
 6. Thesystem of claim 5 further comprising a clock signal conditionerelectrically coupled to the drive signal generator to receive the drivesignal, the clock signal conditioner including a phase shifterconfigured to output a shifted signal out of phase with the drivesignal.
 7. The system of claim 6 wherein the phase shifter of the clocksignal conditioner is configured to output the shifted signal 90° out ofphase with the drive signal and wherein the clock signal conditionerfurther comprises a first comparison circuit with inputs coupled toground and to the phase shifter to receive the shifted signal, andwherein the signal processor includes a second comparison circuit and aflip-flop, the second comparison circuit with inputs coupled to thereference coil and the sense coil and an output coupled to an input ofthe flip-flop, the flip-flop having a clock input coupled to an outputof the first comparison circuit.
 8. The system of claim 1 furthercomprising an electro-dynamic machine mechanically coupled to the shaft.9. The system of claim 1 wherein the signal processor includes one ofthe following: a comparison circuit and a microcontroller.
 10. Thesystem of claim 1 wherein the turns of wire of the reference coil arewound around the first axis and the shaft passes through the entirereference coil along the first axis during all of the bi-directionallinear travel of the shaft such that the shaft magnetically engages thereference coil to a degree independent of shaft position during thebi-directional linear travel of the shaft.
 11. The system of claim 10wherein the signal processor is configured to report the at least oneposition of advancement as a null position of the bidirectional travelof the shaft.
 12. The system of claim 1 wherein the turns of wire of thedrive coil extend around the first axis and the shaft passes through theentire drive coil along the first axis during all of the bi-directionallinear travel of the shaft.
 13. The system of claim 1 wherein the turnsof wire of the reference coil extend around the first axis and the shaftpasses through the entire reference coil along the first axis during allof the bidirectional linear travel of the shaft, and wherein the turnsof wire of the drive coil extend around the first axis and the shaftpasses through the entire drive coil along the first axis during all ofthe bidirectional linear travel of the shaft such that the shaftmagnetically engages the reference coil to a degree independent fromposition of the shaft during the bi-directional linear travel of theshaft.
 14. A system comprising: a first member with an internal boreextending along an axis; a second member, at least a portion of whichbeing positioned in the internal bore of the first member and movabletherein in a bidirectional travel along the axis; a signal generatorconfigured to generate a signal; a drive coil of wire wound a number ofturns around a first portion of the first member, the signal generatorelectrically coupled to the drive coil to energize the drive coil toproduce an magnetic flux; a reference coil of wire with first and secondends, the reference coil wound a number of turns around a second portionof the first member; a sense coil of wire with first and second ends,the sense coil wound a number of turns around a third portion of thefirst member; and a signal processor having inputs electrically coupledto the reference coil and the sense coil to determine a differencebetween a parameter with respect to the reference coil compared with aparameter across the first and second ends of the sense coil to identifywhen the second member is at a predetermined position within theinternal bore of the first member.
 15. The system of claim 14 whereinthe parameter with respect to the reference coil is a voltage across thefirst and second ends of the reference coil and the parameter withrespect to the sense coil is a voltage across the first and second endsof the sense coil.
 16. The system of claim 14 wherein the drive coil hasa first end coupled to the signal generator.
 17. The system of claim 16wherein the drive coil has a second end coupled to ground.
 18. Thesystem of claim 14 wherein the first ends of the reference coil and thesense coil are coupled to the inputs of the signal processor.
 19. Thesystem of claim 18 wherein the second ends of the reference coil and thesense coil are coupled the ground.
 20. A system comprising: a memberpositioned along an axis; a signal generator configured to generate asignal; a first coil of wire electrically coupled to the signalgenerator configured to generate an magnetic flux; a second coil of wirepositioned to be in the magnetic flux; a third coil of wire positionedto be in the magnetic flux; and a signal processor having inputselectrically coupled to the second coil and the third coil to determinea difference between a parameter with respect to the second coilcompared with a parameter with respect to the third coil to identifywhen the member is at a predetermined position along the axis.
 21. Thesystem of claim 20 wherein the parameter with respect to the second coilis a voltage across the second coil and the parameter with respect tothe third coil is a voltage across the third coil.
 22. A system forcoupling with a signal generator configured to generate a signal, thesystem comprising: a first coil of wire configured for electricalcoupling to the signal generator to generate an magnetic flux; a secondcoil of wire positioned to be in the magnetic flux; a third coil of wirepositioned to be in the magnetic flux; and a signal processor havinginputs electrically coupled to the second coil and the third coil todetermine a difference between a parameter with respect to the secondcoil compared with a parameter with respect to the third coil toidentify when a member is at a predetermined position.
 23. The system ofclaim 22 wherein the parameter with respect to the second coil is avoltage across the second coil and the parameter with respect to thethird coil is a voltage across the third coil.
 24. A system for couplingwith a signal processor, the system comprising: a signal generatorconfigured to generate a signal; a first coil of wire electricallycoupled to the signal generator configured to generate an magnetic flux;a second coil of wire positioned to be in the magnetic flux andconfigured for coupling with the signal processor; and a third coil ofwire positioned to be in the magnetic flux and configured for couplingwith the signal processor.
 25. A method comprising: generating a drivesignal having a periodically varying parameter; energizing a drive coilwith the drive signal to produce a time varying magnetic flux; exposinga reference coil to the magnetic flux to induce a first parameter withrespect to the reference coil; exposing a sense coil to the magneticflux to induce a second parameter with respect to the sense coil;inserting a portion of a member into the sense coil, the member havingmagnetic permeability greater than the magnetic permeability of a mediumthat at least partially surrounds the member; moving the member in alinear direction along the sense coil to vary the amount that the memberis inserted into the sense coil to change amplitude of the secondparameter with respect to the sense coil; and comparing the firstparameter with the second parameter to identify when the member isinserted a predetermined amount into the sense coil.
 26. The method ofclaim 25 wherein the periodically varying parameter is a periodicallyvarying voltage, the first parameter is a voltage across the referencecoil and the second parameter is a voltage across the sense coil. 27.The method of claim 26 wherein the winding includes positioning thedrive coil along the longitudinal axis of the cylinder between thereference coil and the sense coil.
 28. The method of claim 25 whereingenerating the drive signal is done according to one of the followingfunctions: a square wave function and a sine wave function.
 29. Themethod of claim 28 further comprising generating a shifted signal 90°out of phase and squared from the drive signal and wherein the comparingis done after the shifted signal exceeds an amplitude.
 30. The methodsystem of claim 25 wherein the comparing further includes identifyingwhen the member has been inserted additional predetermined amounts intothe sense coil.
 31. The method of claim 25 wherein the comparing is forthe predetermined amount of insertion of the member into the sense coil.32. A method comprising: generating a signal; energizing a drive coil toproduce an magnetic flux; inducing a parameter with respect to areference coil with the magnetic flux; inducing a parameter with respectto a sense coil with the magnetic flux; and determining a differencebetween an amount of the parameter with respect to the reference coilcompared with the parameter with respect to the sense coil to identifywhen a member has been inserted a predetermined amount into a volume.33. The method of claim 32 wherein the parameter with respect to thereference coil is a voltage across the reference coil and the parameterwith respect to the sense call is a voltage across the sense coil.
 34. Amethod comprising: generating a signal; applying the generated signal toa first coil; and determining a difference between a parameter withrespect to a second coil compared with a parameter with respect to athird coil to identify when a member is at a predetermined position. 35.The method of claim 34 wherein the parameter with respect to the secondcoil is a voltage across the second coil and the parameter with respectto the third coil is a voltage across the third coil.
 36. A methodcomprising: inducing a parameter with respect to a sense coil; inducinga parameter with respect to a reference coil; inserting a member into athe sense coil a predetermined amount; and comparing the parameter withrespect to the sense coil to the parameter with respect to the referencecoil to determine that the member has been inserted into the sense coila predetermined amount.
 37. The method of claim 36 wherein the parameterwith respect to the sense coil is a voltage across the sense coil andthe parameter with respect to the reference coil is a voltage across thereference coil.